As integrated circuit (IC) technology continues to advance, and operational clocking speeds increase, circuits require more power more quickly. The traditional concept behind using capacitors to decouple ICs is to give each IC a localized reservoir of high-frequency energy. In essence, local capacitors help to “decouple” associated ICs from the main power supply, decreasing the magnitude of high frequency ripple or sag that appears on the main power bus. One or more bulk decoupling capacitors on a circuit board, in turn, typically operate to replenish each of the local capacitors.
Often circuit designers seek to use a specific amount of capacitance for a circuit, but only certain standard values are available. Using a fixed number of selected values to satisfy this need often results in over-design. In addition, the use of several different capacitors throughout a particular design to meet various needs can increase the overall circuit cost. A further difficulty is the amount of real estate required to support the use of multiple capacitors.
Capacitors are also not ideal circuit elements. In fact, a capacitor is typically modeled as a series circuit, with a total series impedance of CCAP+ESRCAP+LCAP, corresponding to the sum of the capacitive, resistive, and inductive components which make up a real-world capacitor, respectively. Some available fabrication methods allow control over the specific capacitance value, and even the equivalent series resistance. This is useful when many thousands of capacitors are made on an assembly line, but not for individual circuits, which may require fine-tuning of the individual capacitance and series resistance values to ensure optimal performance.
When a capacitor is placed on a circuit board, the inductance of the traces and other connecting circuitry between the capacitor and the associated chip may further affect circuit performance at high clock speeds. The capacitor and connecting circuitry (traces) that lead to it can form a current loop that operates as an antenna, transmitting radio frequency interference generated by fast transients. Thus, bypass capacitors can do their job most efficiently when mounted in close proximity to the associated IC pins that draw such transient currents.
In addition to low series inductance in a capacitor, it's usually desirable to have a low effective series resistance, which goes hand-in-hand with a low dissipation factor. However, sometimes a very low series resistance can provoke unexpected problems in the form of resonance, especially when the value of an associated motherboard series resistance is not matched to the sum of capacitor series resistance and the resistance of traces forming interconnecting circuitry. When repetitive pulses excite the resonator formed by a low equivalent series resistance capacitor and the associated motherboard, high-amplitude ringing can result, producing an exceedingly noisy supply bus. The typical solution is to place electrolytic capacitors across the bus to damp the ringing, which is costly and uses a large amount of circuit board real estate. A better solution would be to somehow increase the series resistance of the bypass capacitor, without adding additional capacitance or inductance.